The present invention relates to a treatment object boat used in a vertical heat treatment apparatus.
In general, a quartz wafer boat is used for transporting semiconductor wafers to within a heat treatment apparatus and typically includes a plurality of rods secured to the side plates, and in each rod a stopper means is formed for supporting the wafers.
However, when various types of heat treatment, such as oxidation, diffusion, annealing and CVD processing are performed on the semiconductor wafers, major technical problems are encountered that include maintaining uniformity in the temperature distribution on the wafer surface, treatment within the wafer surface, and thickness of the various types of formed films such as oxidation film, in addition to preventing wafer slippage, inversion and other abnormalities. Furthermore, the trend toward large diameter wafers in recent years has rendered resolving these technical problems increasingly difficult.
Also, high speed heat treatment apparatus is being used in recent years, for the purpose of improving the uniformity and quality of the film formed on the wafer. By utilizing such a high speed heat treatment apparatus and performing heat treatment with the following type of cycle, the film uniformity and quality can be improved.
The temperature in the furnace is reduced when the wafers are inserted therein, as the atmosphere is mixed when the wafers are inserted, this is purged with a high purity gas, the temperature is then raised at high speed for performing such treatments as oxidation, diffusion and CVD. After treatment, the temperature is reduced at high speed.
The purpose of using high speed temperature increase and decrease in this manner is to raise through-put, but this encounters the following problems.
Ordinarily, with these various types of heat treatment apparatus, since the heat source is arranged at the perimeter of the wafer, the temperature change at the wafer circumference is faster than at the inner portions. Consequently, when high speed temperature increase and decrease are performed, a large temperature differential is produced in the wafer diameter and due to the resulting thermal distortion, numerous problems occur, such as wafer slippage and inversion.
Previously, as a means for resolving this problem, a type of boat known as a ring boat has been used. With this ring boat as the wafer supporting means, the heat capacity of the perimeter was made larger than that of the central portion, for example, by using a ring shaped tray. By using this type of shaped tray, when heat treating the wafers loaded on the tray, it is possible to prevent the temperature change at the perimeter from occurring faster than at the center, and maintenance of uniform temperature change in the wafer surface is possible.
Using this type of ring boat, two shifting methods were previously employed for shifting the wafers from the wafer cassette to the boat, which are indicated in FIGS. 13 and 14, and 15 and 16.
The conventional shifting method indicated in FIGS. 13 and 14 comprises a shifting fork 90 for shifting the wafer 18 and a raising fork 92 provided with a plurality (for example 3) of raising means 94 for raising the wafer from the bottom. This shifting means shifts the wafers 18 from the wafer cassette to the wafer boat in the following manner.
The wafer 18 is removed from the wafer cassette by the shifting fork 90. This shifting fork 90 which is holding the wafer 18 and the raising fork 92 are inserted from the side of the ring tray 96, which comprises the boat, as indicated in FIG. 13. Then as indicated in FIG. 14, the wafer 18 is raised by the raising fork 92, and by this the wafer 18 is supported by the projections 94, then the shifting fork 90 is retracted from the boat. Finally, the raising fork 92 is lowered and the wafer is loaded on the ring tray 96.
Another conventional shifting method is indicated in FIGS. 15 and 16 which comprises a shifting fork 90 for shifting the wafer and projections 100 for raising the wafer 18 from the bottom, and possesses a vertically movable raising base 98. In this system, the wafer 18 is shifted from the wafer cassette to the boat in the following manner.
The wafer 18 is removed from the wafer cassette by the shifting fork 90. As indicated in FIG. 15, this shifting fork 90 which is holding the wafer 18 is inserted from the side of the ring tray 96 which comprises the boat. Next, as indicated in FIG. 16, the raising base 98 which passes through the ring tray 96 center hole is raised, supporting and raising the wafer 18. Afterwards, the shifting fork 90 is retracted. Finally, the lifting base 98 is lowered, and the wafer 18 is loaded on the ring tray 96.
However, as mentioned above, the major technical problems encountered by regarding this type of heat treatment apparatus include maintaining uniform temperature distribution within the wafer surface, maintaining uniform film thickness and preventing wafer slippage and inversion. In addition to these, there are other serious technical problems such as the following.
There are the requirements to treat as large a number of wafers as possible during each batch treatment cycle and to the extent possible shorten the treatment time for each batch treatment cycle. Meeting these requirements is essential for improving wafer batch treatment efficiency and wafer production through-put, in addition to reducing the wafer production cost.
However, these requirements cannot be adequately fulfilled by the methods indicated in FIGS. 13-16. In the method of FIGS. 13 and 14, since the raising fork 92 with projections 94 needs to be inserted from the side between ring trays 96, a small pitch cannot be used for the ring trays 96. Consequently, treating a large number of wafers at each batch treatment cycle is extremely difficult. Especially difficult is conserving space in the height direction in a high vertical type heat treatment furnace.
Also, in the method of FIGS. 15 and 16, the wafers can only be exchanged one at a time. The wafers are shifted to the wafer boat in sequence from the top of the boat. Likewise, the wafers are removed from the boat one wafer at a time in sequence from the bottom. Consequently, shortening the exchange time per batch treatment cycle is extremely difficult. In addition, after loading all the wafers, operations such as inserting a dummy or monitor wafer at a desired position are impossible.